Nature of the Dirac gap modulation and surface magnetic interaction in axion antiferromagnetic topological insulator [Formula: see text].

ABSTRACT

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator [Formula see text] and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9-35 K), light polarizations and photon energies. We have distinguished both large (60-70 meV) and reduced ([Formula see text]) gaps at the DP in the ARPES dispersions, which remain open above the Neél temperature ([Formula see text]). We propose that the gap above [Formula see text] remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for the "large gap" sample and apparently significantly reduced effective magnetic moment for the "reduced gap" sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calculations surface structural modification can result in a significant modulation of the DP gap.